1. Field of the Invention
This invention relates to a pneumatic tire suitable for running on snow and ice roads, and more particularly, it is concerned with a pneumatic tire suitable for running on snow and ice roads, which has improved performances by increasing the tread-gripping force of the tread portion on snow or ice road surface. In this case, the snow and ice roads include both general snow and ice roads at a temperature of at least -20.degree. C. (hereinafter referred to as snow and ice roads) and completely frozen ice roads at a temperature of at most -20.degree. C. in severe cold regions (hereinafter referred to as completely frozen roads). The following illustration is according to this definition.
2. Description of the Prior Art
There have heretofore been proposed various pneumatic tires suitable for running on snow and ice roads and completely frozen roads, but their performances are not necessarily satisfactory and further improvement thereof is still desired. In this respect, various efforts to a study of the materials of a tread portion have heretofore been made in order to increase the tread-gripping force on snow-covered or ice-covered road surfaces. For example, it is well-known from C. S. Wilkinson; RCT 27 255 (1954), F. S. Conant; RCT 22 863 (1949), etc. that the hardness of rubber at low temperature is lowered. As is described in W. G. Norich; "The friction of Polymer on Ice", Journal of the IRI, October page 192 (1972), Desmond Moore; "The Friction of Pneumatic Tires" (1975), etc., it is known that butadiene rubber has an excellent tread-gripping force. However, butadiene rubber shows lowered performances on completely frozen roads at lower than -20.degree. C. so that a sufficient tread-gripping force cannot be obtained, which is considered due to that butadiene rubber crystalizes at a low temperature such as -20.degree. C. or lower to raise its modulus of elasticity as well-known in the art.
When using natural rubber or polyisoprene rubber in the rubber composition of a tread portion, the tread-gripping force on snow and ice roads is improved, but that on wet roads is disadvantageously lowered. For the purpose of mitigating the lowering of the tread-gripping force on wet roads, it has also been proposed to use a large amount of styrene-butadiene copolymer rubber, butyl rubber or carbon black as disclosed in A. C. Bassi; RCT 38 112 (1965), D. Bulgin, G. D. Hubberd and M. H. Walters; Proc. 4th Rubber Tech. Conf. London 193 (1962), etc. In this case, however, the performances on snow and ice roads and rolling resistance are unfavourably lowered.